EXISTING ORGANISMS— PHYSIOLOGY 95 



metamorphosis before attaining the normal growth, while the 

 Axolotl, a salamander which normally remains aquatic, may be 

 caused to develop into a terrestrial animal tlirough the usual am- 

 phibian metamorphosis by the administration of the same sub- 

 stance. Likewise, the symptoms of cretinism may be corrected and 

 normal development induced in infants by supplying thyroid ex- 

 tract secured from domestic animals. 



The part played by insulin, a hormone secreted by the islands 

 of Langerhans in the pancreas, has recently been given great pub- 

 licity because of its isolation and the discovery of its effect on the 

 metabolism of carbohydrates. Insufficiency of this secretion 

 results in the disease diabetes, whose effects are now minimized or 

 completely abated by the administration of the commercially 

 extracted secretion of other animals. 



These two are familiar examples, but are duplicated by the 

 behaviour of all the other endocrine glands. The interspecific 

 effectiveness of hormones is so dependable that a considerable 

 number are now commercially available, all extracted from the 

 glands of domestic animals, and are used extensively in endocrine 

 therapy. The one limitation is not their lack of potency, but our 

 incomplete knowledge of their action. 



Blood Tests. One evidence of physiological relationship has 

 received much attention in the literature of evolution, namely 

 Nuttall's famous precipitin tests for blood. The physiological 

 basis for these tests is treated in Nuttall's book from which we 

 ma}' draw. 



Immune Sera. Immunizing properties of l)lood serum removed 

 from animals which had developed natural immunity from such 

 diseases as diphtheria and tetanus were noted late in the nine- 

 teenth century. Ehrlich then experimented with the toxic sub- 

 stances ricin and abrin. He found that animals treated with in- 

 creasing doses of these poisons developed increased tolerance, or 

 immunity, and that the blood sera of these animals neutralized 

 the poisons in vitro, and, of course, when injected into other indi- 

 viduals rendered them immune from the effects of the poisons. 

 He proved also that a serum capable of neutralizing ricin had no 

 effect upon abrin and vice versa. EhrUch concluded that definite 

 compounds were produced in the blood in response to the poisons. 

 These he called antitoxins, or antibodies. As Nuttall points out 

 "we now know that normal serum contains a number of anti- 



